1. Field of the Invention
The present invention generally relates to air filter devices and more particularly to fabric air screens or filters covering exhaust air ducts or ports or hatches in storage containers for storing aggregate materials.
2. Background of the Invention
Dry bulk storage bins or units are used on oil and gas drilling sites to store dry bulk aggregate such as frac sand. The aggregate—generally silica sand or SiO2—is typically loaded into the storage bins from pneumatic trailers used to transport the aggregate to the drilling site. When transferring the aggregate to the storage bins, inspection hatches in the ceiling or wall of the storage bin may be routinely left open during the loading process to exhaust the air used to transport the aggregate from the pneumatic trailer to the storage unit. This aggregate is frequently accompanied by substantial amounts of silica dust that may be released to the atmosphere through the inspection hatches as the aggregate is transferred between containers. Release of the silica dust into the air can be a significant problem, not only in environmentally sensitive areas but also because silica dust can be a contributor to lung disease. Reducing exposure to the silica dust, can potentially and materially reduce the hazard.
Conventional dust filtering or collection devices and methods are disclosed in the prior art for use in vacuum cleaning, air filtering, and dust collecting systems. Vacuum cleaners typically pull air or gases laden with particulate matter through a porous bag used to collect the particulate matter and contain it therein for later disposal. Ventilation systems place an air filter element in the active, i.e., inlet air path, usually positioned at an inlet to the system. Household HVAC systems are one example of this type of ventilation system. In other systems air from a dust-producing environment is passed through an air filter element placed in an air outlet as the air circulating through that environment is exhausted, similar to a typical vacuum cleaner. The filter element in such ventilation systems must typically be replaced or removed frequently for cleaning to dispose of the dust retained by the filter element. The foregoing systems have the common feature that the dust is collected so that it can be removed and disposed as waste.
Industrial systems for removing dust, a relatively light-weight, low density substance, such as found in manufacturing and material processing plants use so-called “bag houses,” typically configured as housings suspended over a dust-collecting hopper or other container. These housings typically contain a plurality of bag-type or pleated paper filter elements to provide a large amount of filtering surface area within a limited space. In operation, dust-laden air is conveyed into the housing which separates the dust from the air via the plurality of filter elements wherein the air is conveyed horizontally or downwardly through these elements and the dust particles deposited in the bag or in the filter element, or in a separate container such as a hopper or bin below the filter element. However, such apparatus tends to be complex and expensive, requires substantial maintenance to clean or replace the plurality of elements, and often is built into the facilities or vehicles or machinery used to process, transport, store, and convey aggregate matter from one place to another. Other examples include dust-collecting bags suspended from horizontally oriented ports. These have the disadvantage that the bags tend to fill up with the particulate matter the bags are intended to separate from the air flow, requiring frequent replacement, emptying, or back flow systems to maintain full air flow through the apparatus. Here again, the dust is collected in a filter or container that must be removed so that the collected dust can be discarded. Moreover, such filtering apparatus is generally not readily adaptable to equipment used in oil and gas drilling operations and the like without substantial modification.
As is well-known, the equipment used during drilling for oil and gas is designed to be mobile so that it can be moved from site to site. In the relatively new field of hydraulic fracturing technology currently in widespread use in such activities, water mixed with sand and certain chemicals is injected under very high pressure into drilled well passages deep into rock strata bearing deposits of petroleum hydrocarbons. This ‘fracing fluid” is used to fracture the rock and support the cracks created by the fracturing with the particles of sand or ceramic beads (called “propants”) in an open position to enable release of the hydrocarbons bound within the rock strata. The handling of the frac sand on site to be mixed with the water creates a need for a method and apparatus for handling these materials without releasing into the atmosphere the silica dust or other fine particulate matter that accompanies the frac sand while conveying it from transport vehicles to storage facilities at the drilling site.
The facilities at a drilling site for storing the frac sand aggregate are typically portable enclosures that have an inlet port in the side of the enclosure and an inspection hatch in the ceiling of the enclosure. Some enclosures have multiple compartments and inspection hatches. In use, a conveyor duct is connected from the discharge port of the transport vehicle to an inlet of the storage facility or unit. The aggregate, introduced into the duct from the transport vehicle, is conveyed through the duct in a high-velocity air stream produced by a high capacity air pump. The high capacity is required to transfer substantial quantities of the relatively heavy frac sand though duct work and into a storage unit. Substantial volumes of air laden with the heavy aggregate sand is thus conveyed into the compartments of the storage units. This high velocity air stream is provided under high pressure by the high capacity air pump disposed at the air inlet to the conveyor duct and into the compartment (s) of the storage unit. The inspection hatches of the storage facility are typically left open during the transfer operation to prevent the build up of back pressure in the conveyor system, which back pressure would cause poor efficiency and lengthen the time to the transfer of the sand into the storage facility. The problem created with this practice is that substantial amounts of silica dust is exhausted into the atmosphere along with the air used to convey the sand, causing undesirable dust pollution, loss of some of the sand available for use in the fracing process, and potentially creating a health hazard. The conventional systems described above for removing dust from a work area described above are either not readily adapted to the mobile equipment generally used at drilling sites or are generally complex and expensive, made more so when it is attempted to adapt them to mobile equipment.
What is needed is a simple device or method for screening, filtering or otherwise preventing the release of the dust suspended in the air stream while loading heavy or dense aggregate materials into storage bins or otherwise when transferring such aggregate materials by pneumatic means from one location to another. The device or method must be low in cost, portable, and readily suited to the mobile equipment in use at such mobile drilling facilities.